Ultrasonic energy has been applied to liquids in the past. Sufficiently intense ultrasonic energy applied to a liquid, such as water, produces cavitation that can induce changes in the physiochemical characteristics of the liquid. The subject of sonochemistry, which deals with phenomena of that sort, has grown very much during recent years.
The published material in sonochemistry and related subjects all pertains to batch processes, that is, the liquid solution or dispersion to be treated is placed in a container. The liquid in the container is then stirred or otherwise agitated, and ultrasound is applied thereto. It is then necessary to wait until the desired result, physical or chemical change in the liquid, is achieved, or until no improvement in the yield is observed. Then the ultrasound is turned off and the liquid extracted. In this way liquid does not return to its initial state prior to the treatment with ultrasonic energy. In this respect, the ultrasound treatment is regarded as irreversible or only very slowly reversible.
Far from all industrial processes using liquids are appropriately carried out in batches, as described above. In fact, almost all large-scale processes are based upon continuous processing. The reasons for treating liquids in continuous processes are many. For example, the fact that a given process may not be irreversible, or only slowly reversible, and requires that the liquid be immediately treated further before it can revert to its previous state.
Shock waves external to collapsing bubbles driven onto violent oscillation by ultrasound are necessary for most if not all physiochemical work in liquid solutions. The under-pressure pulses form the bubbles and the pressure pulses compress the bubbles and consequently reduce the bubble diameter. After sufficient number of cycles, the bubble diameter is increased up to the point where the bubble has reached its critical diameter whereupon the bubble is driven to a violent oscillation and collapses whereby a pressure and temperature pulse is generated. A very strong ultrasound field is forming more bubbles, and drives them into violent oscillation and collapse much quicker.
A bubble that is generated within a liquid in motion occupies a volume within said liquid, and will follow the speed of flow within said liquid. The weaker ultrasound field it is exposed to, the more pulses it will have to be exposed to in order to come to a violent implosion. This means that the greater the speed of flow is, the stronger the ultrasound field will have to be in order to bring the bubbles to violent implosion and collapse. Otherwise, the bubbles will leave the ultrasound field before they are brought to implosion. A strong ultrasound field requires the field to be generated by very powerful ultrasound transducers, and that the energy these transducers generate is transmitted into the liquid to be treated. Based upon this requirement, Bo Nilsson and H{dot over (a)}kan Dahlberg started a development of new types of piezoelectric transducer that could be driven at voltages up to 13 kV, and therefore capable of generating very strong ultrasonic fields.
A very strong ultrasonic source will cause a cushion of bubbles near the emitting surface. The ultrasound cannot penetrate through this cushion, and consequently no ultrasound can penetrate into the medium to be treated. The traditional way to overcome this problem is to reduce the power in terms of watts per unit area of emitting surface applied to the ultrasonic transducers. As indicated above, the flow speed of the medium to be treated will require a stronger ultrasound field and therefore an increased power applied to the ultrasonic transducers. The higher the power input is, the quicker the cushion is formed, and the thicker the formed cushion will be. A thick cushion will completely stop all ultrasound penetration into a liquid located on the other side of this cushion. All the cavitation bubbles in this cushion will then stay in the cushion and cause severe cavitation damage to the ultrasound transducer assembly area leading to a necessary exchange of that part of the ultrasound system. This means that little or no useful ultrasound effect is achieved within the substrate to be treated, and that the ultrasound equipment may be severely damaged.
The above-outlined cushion problems also apply to treating bacteria clusters in sludge slurries and treating drainage water from sludge slurries in sewage works that are subjected to ultrasonic treatment. The problems also apply to other processes with ultrasonic treatment of slurries, such as the forming of paper webs, de-inking of recycled pulp and cleaning of polluted soil. They also apply to other processes where liquids are treated with ultrasound, such as treatment of water polluted with solvents, and cleaning of drinking water and sonochemical processes.
One problem with the currently used sludge ultrasonic treatment plants is that the energy consumption is high and the efficiency could be improved. There is a need to solve the problems outline above so that sewage works may use ultrasonic treatment for bacteria in the sludge without encountering the undesirable cushion effect or the low efficiency. The method of treating a sludge slurry of the present invention provides a solution to the problems outlined above.
More particularly, the method of the present invention is for treating a slurry, such as sludge, with an ultrasonic energy without creating the undesirable cushion effect. Movable endless members are provided that are permeable to the liquid part of a sludge slurry and a first ultrasonic transducer is disposed adjacent to a first movable member and a second ultrasonic transducer is disposed adjacent to a second movable member. The slurry is fed in between the two movable members. The transducers generate pressure pulses through the members to form imploding cavitation bubbles in the sludge slurry that have an effect on the bacteria clusters. The cavitation bubbles have a resonance diameter (d5) at the ultrasound frequency used that is greater than a distance (d3) between the first transducer and the first member and a distance (d4) between the second transducer and the second member to prevent the bubbles from imploding between the transducers and the members. By making the distance between the members smaller and smaller along the ultrasonic treatment path, a hydraulic pressure build-up between the members causes a dewatering of the slurry through the members giving a higher and higher dry solids content of the sludge slurry that is favorable for the efficiency of the ultrasonic treatment. The edges of the upper and lower members are pressed together to prevent the sludge from leaving the treatment zone in the cross machine direction. When treating liquids there are wedge formed sidewalls between the members and the edges of the members are pressed towards these sidewalls and the contact areas are water lubricated to minimize friction. The treated sludge may then be pumped to an anaerobic fermentation tank. Biogas can be continuously removed from the sludge by the under-pressure in a degassing pump or other degassing unit in a circulation loop connected to the fermentation tank before any gas bubbles are formed in the fermentation tank. The sludge slurry may again be subject to degassing and ultrasonic treatment before the slurry is sent to a press unit for dewatering.